A gas chromatograph analyzer, as is well known in the art, separates a sample gas into its constituent components by injecting the sample into a carrier gas stream which then moves the sample through a column which contains a stationary liquid supported on a porous packing material. In passing through the column the sample is partitioned into its component compounds which emerge from the column at times dependent upon the compositions of the components. The effluent gas from the column is passed into a detector which senses a change in the characteristic of the gas, such as thermal conductivity, and produces an electrical output accordingly. The detector output is recorded against a time base, usually by means of a strip chart recorder, providing a record of the times elapsing between injection of the sample into the column and the emergence of various components from the column together with peak values indicative of the relative quantities of the components in the sample.
The ability of a gas chromatograph to resolve a sample into its components is dependent in large part upon the speed of injection of the sample into the column and the shape of the sample. Ideally, the sample is in the form of a very narrow plug having flat, sharply defined leading and trailing interfaces with the carrier gas. Conventionally, sample gas is injected into the chromatograph column by an electrically or pneumatically actuated sample valve which momentarily switches the flow path from a carrier gas source into the column inlet so that the carrier gas first passes into a sample chamber and simultaneously switches the sample chamber outlet to the column inlet. After sample injection carrier gas flow is switched back to the column inlet.
The speed of the sample valve operation is obviously limited by its mechanical construction, the inertia of the movable parts thereof and the available actuation force. Limitations upon the opening and closing speed of the sample valve and the valve throttling effect adversely affect the size and shape of the pulse of sample gas entering the chromatograph column, producing generally a broader than desirable sample pulse having interfaces which are distorted by diffusion with the carrier gas. The diffusional boundaries of the sample pulse result in an injection time which is not clearly defined, while excessive breadth of the sample pulse further confuses the precise time of injection of the sample into the column and may introduce a sufficient volume of sample gas into the column to cause saturation of the column stationary phase or saturation of the detector. The resultant chromatogram may then show overlapped peaks which obscure certain components of the sample or complicate the identification thereof. Saturation of the column stationary phase or detector may cause a complete failure of the detector to sense component peaks and further impairs the utility of the analyzer by prolonging the time required to backflush and cleandown the column and the detector.
The present invention eliminates the problems associated with mechanical sample valves in gas chromatograph analyzers by replacing the sample valve with an electronically controlled ion gated which is capable of admitting to the chromatograph column inlet, at a precisely controlled time, a very narrow, sharply defined sample gas pulse of known shape. The sample inlet control of the invention is based upon the principle of an ion mobility detector, an instrument normally used alone for classifying gaseous compounds in accordance with the mobility of ions thereof, which are accelerated by an electric field to a size/mass dependent terminal velocity in a stream of counterflowing drift gas.
A typical ion mobility detector is described in U.S. Pat. No. 3,845,301, there termed a plasma chromatograph, as comprising a gas enclosing cell divided by ion shutter grids into a reaction region, a drift region and a detector region. Sample gas and a readily ionizable reactant gas are introduced into the reaction region where a radioactive source ionizes the reactant gas to produce primary ions. Collisions between the primary ions and sample gas molecules produce product ions. Both the primary ions and the product ions are urged towards the drift region by an electrostatic field established longitudinally of the cell. The ions are blocked from entering the drift region by a first, negatively charged shutter grid. The ion repellant charge is momentarily removed from the first shutter grid and a group of diverse product ions is allowed to enter the drift region. The ions are accelerated through the drift region by an electrostatic field toward the detector region against a counterflowing drift gas stream. In traversing the drift region the product ions become separated into distinct groups which are classifiable by their time of arrival at the detector region. The drift times for the various groups are determined by momentarily removing a negative charge from a second shutter grid located adjacent the ion detector, allowing a particular ion group having a drift time corresponding to the delay between opening the first and second shutter grids to pass to the detector.
The ion mobility detector can provide considerable information concerning the composition of a sample. However, the informatin provided does not characterize the sample components as to charge/mass ratio only, since molecular size has an influence upon the mobility of the ions. To provide further information as to the composition of the sample components, U.S. Pat. No. 3,845,301, referred to above, discloses a plasma chromatograph, i.e. ion mobility detector, arranged to serve as an ion source for a mass spectrograph. The plasma chromatograph operates in the usual manner to produce product ions and to separate those ions into groups according to their mobilities. A particular ion group, selected by the delay time of the opening of the second shutter grid is then passed as an ion beam into the high vacuum of a mass spectrometer where the ions may be more particularly characterized in accordance with the charge/mass ratio thereof.
It is an object of the invention to provide a sample gas inlet control for a gas chromatograph analyzer by which the sample is rapidly injected into the column.
It is a further object of the invention to provide a sample gas inlet control for a gas chromatograph analyzer by which the time of injection of the sample into the chromatograph column is precisely known.
It is still another object of the invention to provide a sample gas inlet control for a gas chromatograph analyzer by which the physical form of the sample injected into the chromatograph column is known.
It is yet another object of the invention to provide a sample gas inlet control for a gas chromatograph analyzer by which the components of a sample gas can be separated as to their mobility characteristics for selective injection of such separated components into the chromatograph column.